ΔE 12.2, fair: the same colour family, but not a straight swap. Flat Earth is noticeably darker than Noble Steed Brown.
Can I use Flat Earth instead of Noble Steed Brown?
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 12). Flat Earth is noticeably darker than Noble Steed Brown.
Can I use Noble Steed Brown instead of Flat Earth?
The same difference applies the other way round: Noble Steed Brown is noticeably lighter.
ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.